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Carbohydrate intolerance
Last reviewed: 05.07.2025
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Carbohydrate intolerance is the inability to digest certain carbohydrates due to a deficiency of one or more intestinal enzymes. Symptoms of carbohydrate intolerance include diarrhea, bloating, and flatulence. Diagnosis is based on clinical signs and an H2 breath test. Treatment of carbohydrate intolerance involves eliminating disaccharides from the diet.
[ What causes carbohydrate intolerance?
Enzyme deficiencies can be congenital, acquired (primary), or secondary. Congenital deficiencies are rare.
Acquired lactase deficiency (primary adult hypolactasia) is the most common form of carbohydrate intolerance. High lactase levels are seen in newborns due to the need to digest milk; in most ethnic groups (80% of blacks and Hispanics, almost 100% of Asians), lactase levels decrease after the period of breastfeeding, preventing older children and adults from digesting significant amounts of lactose. However, 80-85% of northwestern Europeans produce good lactase throughout life, allowing them to digest milk and dairy products. It is unclear why more than 75% of the world's population is deficient in this enzyme.
Secondary lactase deficiency is associated with conditions that damage the small intestinal mucosa (e.g., celiac disease, tropical sprue, acute intestinal infections). In infants, temporary secondary disaccharidase deficiency may complicate intestinal infections or abdominal surgery. Recovery from the disease is accompanied by an increase in enzyme activity.
What happens when you have carbohydrate intolerance?
Disaccharides are normally broken down into monosaccharides from disaccharides [e.g., lactase, maltase, isomaltase, sucrase (invertase)], localizing to the brush border of enterocytes in the small intestine. Undigested disaccharides cause an increase in osmotic pressure, which attracts water and electrolytes into the intestinal lumen, causing watery diarrhea. Bacterial fermentation of carbohydrates in the colon causes gas formation (hydrogen, carbon dioxide, and methane), leading to severe bloating, flatulence, and abdominal pain.
Symptoms of Carbohydrate Intolerance
The symptoms of carbohydrate intolerance are similar for all disaccharidase deficiency conditions. A child with lactose intolerance develops diarrhea after ingesting large amounts of milk and may fail to gain weight. Adults may have watery diarrhea, bloating, excessive gas, nausea, abdominal rumbling, and intestinal cramping after ingesting lactose. Patients notice this early and avoid dairy products. Symptoms of carbohydrate intolerance usually begin after ingestion of the equivalent of 8 to 12 ounces of milk. Diarrhea may be severe enough to cause other nutrients to be excreted before they are absorbed. Symptoms may be similar to irritable bowel syndrome, requiring differential diagnosis.
Where does it hurt?
What's bothering you?
Diagnosis of carbohydrate intolerance
Lactose intolerance is usually diagnosed by careful history taking, supported by dietary patterns. Patients usually have a history of intolerance to milk and dairy products. The diagnosis can be suspected if the stools are acidic (pH < 6) in the form of chronic or intermittent diarrhea and can be confirmed by an H2 breath test or lactose tolerance test.
In the H2 breath test, the patient takes 50 g of lactose orally, and the H2 formed during the metabolism of undigested lactose by microflora is measured by the device during breathing 2, 3 and 4 hours after eating. In patients with significant manifestations of the disease, the increase in H2 reaches more than 20 mmol above the baseline value. The sensitivity and specificity of the study are more than 95%.
The lactose tolerance test is less specific. Lactose (1.0-1.5 g/kg body weight) is given orally. Blood glucose is measured before and 60 and 120 minutes after the meal. Patients with lactose intolerance develop diarrhea, bloating, and discomfort within 20-30 minutes and blood glucose levels do not rise above 20 mg/dL (< 1.1 mmol/L) from baseline. Low lactase activity in jejunal biopsy may confirm the diagnosis, but endoscopy is difficult to obtain a tissue sample.
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Treatment of carbohydrate intolerance
Carbohydrate intolerance is easily controlled by eliminating sugars from the diet that cannot be absorbed by the intestine (e.g., a lactose-free diet in the case of lactase deficiency). However, since the degree of lactose malabsorption varies greatly, many patients can take up to 12 ounces (18 g) of lactose-containing milk daily without developing symptoms. Yogurt is usually tolerated because it contains significant amounts of lactase, produced by the Lactobacilli it contains.
For patients who wish to consume milk, a method of pre-treatment of lactose by adding ready-made lactase to milk has been developed, which has allowed such patients to consume milk. The addition of the enzyme should only be a supplement, but not used instead of a restrictive diet. Patients with lactose intolerance should additionally take calcium supplements (1200-1500 mg/day).
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